WASHINGTON (Reuters) - Brent Scowcroft, who was national security adviser to two Republican U.S. presidents, on Wednesday endorsed Democrat Hillary Clinton for president over Donald Trump, in a snub of the Republican Party's likely nominee by a prominent member of its security wing.

"The presidency requires the judgment and the knowledge to make tough calls under pressure," the Republican elder statesman said in a statement. "I believe Hillary Clinton has the wisdom and experience to lead our country at this critical time."

Scowcroft, 91, served as national security adviser under Presidents Gerald Ford and George H. W. Bush. Ford was president from 1974 to 1977 and Bush from 1989 to 1993.

He joined a number of well-known Republicans defecting due to the prospect of a Trump presidency, including Richard Armitage, deputy secretary of state under President George W. Bush.

Armitage gave Clinton, 68, who was secretary of state under Democratic President Barack Obama, his backing last week. Trump "doesn't appear to be a Republican, he doesn't appear to want to learn about issues. So, I’m going to vote for Mrs. Clinton," Armitage told Politico at the time.

The Trump campaign did not immediately respond to a request for comment on Scowcroft's endorsement of Clinton.

Trump, 70, who has never held elected office, often boasts on the campaign trail his relative newness to politics, appealing to voters disillusioned by the current political climate.

Despite that facet of his appeal - which helped Trump beat 16 rivals in the campaign's primary phase to become his party's presumptive nominee - establishment Republicans worry that his inexperience and brash style will alienate foreign allies and other international partners.

Among his proposals, Trump has criticized NATO, a cornerstone of U.S. security policy, and said he would consider letting Japan and South Korea develop their own nuclear weapons instead of relying on the United States for protection against North Korea and China.

Trump has said he would strengthen national security, but has offered few details about his plans, other than his proposals to build a wall at the U.S.-Mexican border and for a temporary ban on Muslims entering the United States.

Many national security experts have expressed alarm over the proposed ban and have warned that Trump’s fiery rhetoric on the campaign trail is fueling extremism.